<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 200px;
      height: 100px;
      background-color: pink;

      animation: change 1s;
    }

    /* @keyframes change {
      from {
        width: 200px;
      }
      to {
        width: 800px;
      }
    } */

    /* 百分比：表示的意思是动画时长的百分比 */
    @keyframes change {
      0% {
        width: 200px;
        height: 100px;
      }

      20% {
        width: 300px;
        height: 300px;
      }

      100% {
        width: 800px;
        height: 500px;
      }
    }
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>